If you look more carefully into the second sentence he describes the fact that the cooldown can never hit 0. To make an example:
You want to use a spell with a 10 sec cooldown.
You have a legendary affix reducing the cooldown with 20%. That will then shorten the cooldown to 8 seconds.
You then also have 3 items equipped with 10% CDR each. To get the cooldown after each use we say. 10*0.8*0.9*0.9*0.9 which is 5.8 seconds, and not the 5 seconds it would've been if you said 20+10+10+10.. 10*0.5 which is 5 seconds.
With this sort of diminishing returns you would never be able to hit the 0 seconds after each use. Well only if you miraculously get an item with 100% CDR
This is purely my understanding of it. Might be completely off.